1. Field
Embodiments of the disclosure relate generally to the field of electro-mechanical actuators and more particularly to embodiments for releasing a segmented drive coupling that disengages the ball screw from the actuator gear.
2. Background
Actuators provide mechanical force for moving or controlling elements of a mechanical structure. Aircraft, for example, employ actuators for control surfaces such as the rudder and elevators in the empennage of the aircraft and flaps, spoilers and ailerons in each wing. Actuators are additionally employed in the fuselage to open and close the doors that cover the landing gear bays, to raise and lower the landing gear and to deploy engine control thrust reversers for supplementing brakes during deceleration of the aircraft on landing. Actuators are required for similar use on marine and land vehicles for various mechanical systems.
In addition to uses in aircraft and other vehicles, actuators are used in electro-mechanical devices such as computer disk drives to control the location of the read/write head by which data is stored on and read from the disk. Actuators are used in robots, i.e., in automated factories to assemble products. Actuators operate brakes on vehicles; open and close doors; raise and lower railroad gates and perform numerous other tasks in various fields of use.
Hydraulic actuators have been historically used for many applications including aircraft flight controls and subsystems such as landing gear. However, electrically driven actuators which avoid the requirement for plumbing to distribute and control a pressurized working fluid are being used in larger numbers of applications to replace hydraulic actuators. For example, in an airplane, a pump that generates high-pressure working fluid and the plumbing required to route the working fluid may add weight and increase design complexity because of the hydraulic lines. Additionally, connectors on the lines may provide multiple failure points which are avoided with electrical actuators. Electric actuators, which are powered and controlled by electric energy, require only wires to energize and control the unit.
Electro-mechanical actuators (EMA's) are devices that are responsible for moving a mechanical device, and are controlled electrically using sensors for feedback. Linear EMA's are now found in a wide variety of industrial, scientific, and commercial applications, and are used where thrust, speed or position must be controlled. In general, linear EMA's are self-contained systems that convert rotary motion from a motor to linear motion. The actuators are typically operated by an electric motor. Linear motion systems driven by rotating electric motors often employ screw drive systems where the motor rotates a ball screw, lead screw or acme screw, which translates the torque provided by the motor into extension or retraction force through or along the screw.
Some linear EMA's use ball screws for positioning. Such actuators contain a ball screw assembly. Typically, this ball screw assembly consists of a ball screw with a helical groove; a ball nut, also known as the outer race, with an internal groove; and one or more circuits of balls that recirculate in the grooves between the ball screw and the ball nut. This anti-friction design converts torque to linear force as either the ball screw or the ball nut turns and the other component moves in a linear direction.
Other actuators use lead screws in their operation. The lead screw typically uses a plastic or bronze roller nut that slides along the threads of the screw, much like an ordinary nut and bolt. Since there are no rolling elements between the nut and the screw the drive mechanism is simplified.
Windings of electrical motors may be damaged and bearings on motor shafts may wear out. The transmission between the motor and the load, typically the screw and drive arrangement, is also a potential wear point. Such wear may result in mechanical malfunctions may create a jam and prevent motion of the actuator screw. Fault tolerance for these types of malfunctions is therefore necessary in EMA systems to provide equivalent capabilities with redundant hydraulic systems.
It is therefore desirable to provide a release mechanism in the actuator drive train which disconnects the electromechanical actuator from the load upon the occurrence of a jam thus allowing the controlled element to free float or allow redundant actuators freedom of movement in control of the load. In an exemplary aircraft application in the event of a jammed system, the actuator drive train is disengaged allowing a flight control surface to move to a faired position or in the event of a jam in a landing gear actuator disengagement of the EMA drive train allows the gear to freefall to its down and locked position.